(1) Field of the Invention
The invention relates to the general field of plasma etching, more particularly to the antenna effect.
(2) Description of the Prior Art
Dry etching, wherein the etchant is an electrically excited gas, is widely used in the integrated circuit art. In general, the surface to be etched is coated with a patterned layer of photoresist and then exposed to a gaseous plasma of a corrosive gas. In certain situations dry etching in a gaseous plasma offers several advantages over wet etching but plasma etching is also subject to a problem known as the antenna effect. Since the plasma comprises a mix of charged particles there is sometimes a tendency for some of said particles to accumulate on certain surfaces. This, in turn, can lead to damage of certain gates in MOS devices.
How such damage can occur is illustrated in FIG. 1. This shows an isometric cross-section of an FET gate and attached electrode in the process of being formed by etching in a plasma. Polysilicon layer 4 had previously been deposited over the layer of field oxide (FOX) that overlies silicon substrate 1 and is in the process of being shaped to form a gate electrode. This is being effected using plasma etching. Photoresist mask 5 defines the shape that is desired. Because of non-uniformities in the plasma a preponderance of electrons accumulate on the exposed surfaces of layer 4 raising it to a negative potential. This, in turn, leads to a potential difference appearing across gate oxide layer 6. Said potential difference can then lead to damage, either because of dielectric breakdown in 6 or simply as result of tunneling currents through it. Note that, as illustrated by bifurcated arrow 7, for this type of geometry oxide layer 6 acts as a bottleneck for current drawn from a large source and going into a large sink, so that the current density through it can be quite high. An overview of the process can be found in the article by J. P. McVittie in the proceedings of the 1966 "International Symposium on Plasma Process-Induced Damage" pp. 7-10 published by the American Vacuum Society.
Several methods have been reported for monitoring the antenna effect. In particular, these methods provide ways to measure plasma current and voltage. The Stanford Plasma On-wafer Real Time (SPORT) probe has been described in an article by Ma and McVittie in the proceedings of the 1966 "International Symposium on Plasma Process-Induced Damage" pp. 20-23 published by the American Vacuum Society. The intent of SPORT is to minimize perturbation of the plasma while directly measuring charging voltages and currents. FIG. 2 shows a schematic view of a single pad SPORT. Aluminum pad 20 sits on oxide layer 21 which has been formed over part of silicon wafer 22. Polysilicon lead 23 connects to contact pad 24 while pad 25 is connected to silicon wafer 22. Twin lead 12 is connected to 24 and 25 and serves to carry current to the outside of the plasma to oscilloscope (or voltmeter) 13 and bias unit 15 after first passing through low pass filter 14 to remove any RF component. The plasma itself is initiated and sustained between electrodes 16 and 17. A bar magnet 18 may be placed underneath 17 in order to distort the plasma, thereby enhancing the antenna effect.
The SPORT probe is suited for use in a mass-production environment. However, it is an invasive method requiring the running of wires into the plasma chamber.
The CHARge Monitor (CHARM) wafer is a passive monitor based on an Electrically Erasable Programmable Read Only Memory (EEPROM) device. FIG. 3a is a schematic cross-section of CHARM that shows its principal parts. Source/drain 31/32 are controlled from floating gate 35 which receives its charge from charging electrode 36. Due to an asymmetry in the physical shapes of the two surfaces of 35 and 36 that face each other, electrons will flow into 35 from 36 much more easily than in the reverse direction. Thus, the threshold voltage for current flow between 31 and 32 will be a measure of the voltage level to which 35 floated during its exposure to the plasma. By adding current sensing resistor 33 between the substrate and the charging gate (FIG. 3b) current density in the plasma may be determined from the known value of 33 and the voltage that was across it when it was exposed to the plasma (as derived from the EEPROM measurements after removal from the plasma).
The CHARM wafer also has some drawbacks. First, the minimum detectable voltage is about 2.3 volts. Second, the charge stored in the EEPROM decays with time so measurements must be made very soon after exposure to the plasma. Third, fabrication of EEPROMs requires special technology not generally available in most IC fabrication facilities. CHARM has been described in an article by W. Lukaszek et al. in the proceedings of the 1966 "International Symposium on Plasma Process-Induced Damage" pp. 30-33 published by the American Vacuum Society.